Systems and methods for multiple bandwidth electromagnetic geophysical exploration

ABSTRACT

An electromagnetic geophysical exploration system includes a first transmitter-receiver pair that includes a first transmitter configured to transmit a first waveform in a first spectrum and a first receiver configured to sense signals in the first spectrum; and a second transmitter-receiver pair that includes a second transmitter configured to transmit a second waveform in a second spectrum, the second spectrum having a frequency higher than a frequency of the first spectrum, and a second receiver configured to sense signals in the second spectrum; and a control portion. The control portion controls the first transmitter to transmit the first waveform with a first given strength or shape in the first spectrum, and controls the second transmitter to transmit the second waveform with a second given strength or shape in the second spectrum, after a specified time delay.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/305,769, filed Oct. 21, 2016, which is a National Stage of PCTApplication No. PCT/IB2015/000797, filed Apr. 21, 2015, which is relatedto, and claims priority from, U.S. Provisional Patent Application Ser.No. 61/983,299, filed on Apr. 23, 2014, which is incorporated byreference in its entirety for all purposes.

TECHNICAL FIELD

The present disclosure relates generally to geophysical electromagneticexploration and, more particularly, to systems and methods for measuringelectromagnetic responses for geophysical exploration over an extremelywide band of frequencies.

BACKGROUND

Electromagnetic (EM) geophysical exploration methods measure theresponse of subsurface formations to the propagation of naturally orartificially generated electromagnetic fields. Primary electromagneticfields may be generated by passing alternating current or pulsing acurrent through a “transmitter coil” which is an electrically conductingwire or tube which may have an air core or be wrapped around a core madeof some electrical conductor. Use of an alternating current is referredto as frequency domain EM while the use of a pulsed current where thecurrent is applied during an on-period and switched off during anoff-period is referred to as time-domain EM or transient EM. In bothcases, the time-variation of current passing through the transmittercoil produces a response in a large vicinity around the transmittercoil, A transmitter coil may be a small coil made up of many turns ofwire or a large loop of wire with multiple turns. Subsurface formationsrespond to the propagation of time-varying primary electromagneticfields with the generation of secondary electrical currents by theprocess of electromagnetic induction (which is the production of avoltage across a conductor when it is exposed to a varying magneticfield) giving rise to secondary electromagnetic fields. The primary andsecondary electromagnetic fields may be detected by a “receiver.” Areceiver may measure the time-variation of the magnetic field from thesecurrents (for example a coil receiver measuring dB/dt) or may measurethe magnetic field itself (a B-field sensor). Hereinafter, the terms“transmitter coil” and “transmitter” may be used interchangeably and theterms “receiver coil” and “receiver” may be used interchangeably.

The primary electromagnetic field travels from the transmitter coil tothe receiver coil via paths both above and below the surface of theearth. In the presence of a conducting body or earth material such assoils, rocks, ores or other conducting material, the magnetic componentof the electromagnetic field penetrating the subsurface inducestime-varying currents, or eddy currents, to flow in the conducting body.The eddy currents generate their own electromagnetic field (referred toas secondary EM field) that travels to the receiver. The receiver thenundergoes a response to the resultant of the arriving primary andsecondary electromagnetic fields so that the response differs in bothphase and amplitude from the response to the primary electromagneticfield alone. Differences between the transmitted and receivedelectromagnetic fields reveal the presence of the conducting body orconducting material and provide information on the conducting body'sgeometry and electrical properties.

Because electromagnetic fields propagate through air, there is no needfor physical contact of either the transmitter coil or receiver coilwith the earth's surface. EM surveys can thus proceed much more rapidlythan galvanic method surveys, where ground contact is required. Moreimportantly, one or both of transmitter coil and receiver coil can bemounted in or on or towed behind aircraft. Airborne EM methods are usedin prospecting for conductive ore bodies and many other geologicaltargets due to their speed and relative cost-effectiveness.

The electromagnetic response from subsurface materials or bodies isdependent on the electrical conductivity of the material or body.Conductive bodies and other structures such as layers with lowelectrical conductivity exhibit different electromagnetic responses.

Thus, in summary, EM surveying or other geophysical exploration uses theprinciple of electromagnetic induction to measure the electricalconductivity of the subsurface. In the case of a frequency-domain EMsurvey, an alternating electric current of known frequency and magnitudeis passed through a transmitter coil creating a primary EM field in thespace surrounding the coil, including underground. The time-varying EMfields induce a secondary current in underground conductors orstructures which results in an alternating secondary magnetic field thatis sensed by the receiving coil. The secondary field is distinguishedfrom the primary field by a phase lag. The ratio of the magnitudes ofthe primary and secondary currents is proportional to the terrainconductivity. The depth of penetration of the EM field into thesubsurface is governed by the subsurface electrical conductivity andtransmitter excitation frequency and coil separation and orientation.

In the case of a transient EM survey, the same principle ofelectromagnetic induction is used to measure the electrical conductivityof the subsurface. A pulsed electric current of known amplitude andtime-occurrence is passed through a transmitter coil creating a primaryEM field in the space surrounding the coil, including underground. Theeddy currents generated in the ground in turn induce a time-varyingsecondary magnetic field that is sensed by the receiving coil. In theoff-time of the transmitter, the signal magnitude and time-variation ofthe signal magnitude is proportional to the terrain conductivity. In theon-time of the transmitter, the received signal is proportional to theterrain conductivity and to the transmitted primary signal. The depth ofpenetration of the EM field into the subsurface is governed by theterrain conductivity, transmitter power, transmitter excitationfrequency and coil orientation.

The depth of penetration of an EM field depends upon its frequency andthe electrical conductivity of the medium through which the EM field ispropagating. EM fields are attenuated, or weakened, during their passagethrough the subsurface. The amplitude of the EM field decreasesexponentially with depth. The depth of penetration increases as both thefrequency of the electromagnetic field and the conductivity of theground decrease (for example, according to the formulad(m)=503/sqrt(conductivity×frequency)). Consequently, the frequency usedin an EM survey can be tuned to a desired depth range in any particularmedium.

Accordingly, EM survey systems have traditionally been designed withtransmitters to transmit energy in a wide range of frequencies, or be asbroadband, as possible. Similarly, receivers employed in EM surveysystems are designed to measure the EM response for a wide frequencyrange typically sacrificing low-frequency sensitivity for high-frequencyresponse. The high-frequency end of the EM spectrum, or range offrequencies, is used to detect subsurface bodies with large electricalresistivity values (and provide near-surface vertical resolution). Thelow-frequency end of the EM spectrum is used to detect subsurface bodieswith low electrical resistivity values (and provide deeper subsurfaceexploration).

It is impossible, however, with a single system to measure allfrequencies of the spectrum well. To generate energy at highfrequencies, it is necessary in the time domain to employ a transmitterwith a very fast current turn off, which necessitates a low transmitterloop inductance that can be obtained by decreasing the area and numberof turns of the wire loop, which also results in a suppressedtransmitter dipole moment. The small moment also serves to reducehigh-frequency noise by decreasing transients caused by induction ofeddy currents within the conductive survey system components and in theaircraft. To further reduce noise in these system transients, otherstrategies include rigidly connecting the transmitter and receiver, orlocating the receiver such that it is minimum-coupled to thetransmitter. To perform accurate measurement of high-frequencyresponses, it is necessary to measure EM responses of very short timeduration, which requires a receiver coil with a short self-responsewhich requires a small receiver coil turns-area. These designrequirements are in contrast to those for transmitting and measuringlow-frequency responses.

To generate low-frequency energy, it is desirable to maximize thetransmitter dipole moment by increasing the transmitter turns-area andcurrent passed through the wire loop. However, these design parametersincrease the transmitter loop inductance and increase the time forcurrent turn-off (and increasing system transient noise), and aretherefore detrimental for transmitting and measuring high-frequencysignals. Resonance considerations show that a half-sine waveform is themost efficient choice of waveform for converting electric potentialenergy into transmitted current. However, this results in a slow currentturn-off, which is detrimental to generating high-frequency signals. Tomeasure low-frequency signals, it is desirable to increase the voltagegenerated in the receiver coil by having a large receiver coilturns-area. However, this increases the coil self-response and thereforedecreases ability to measure high-frequency signals. Further, to measurelow-frequency signals it is necessary to decrease the effect of receivermotion in the earth's static magnetic field. It is generally desirableto employ soft suspension systems to reduce this effect to the detrimentof the measurement of high-frequency signals. These designconsiderations show that it is impossible to design a single systemwhich transmits and measures high and low-frequency energy well. Thus, aneed has arisen for a system that is capable of operating in bothlow-frequency and high-frequency bandwidths to accurately surveysubsurface properties at both shallower depths (for example,near-surface depths) and deeper depths and to detect targets of a widerconductivity range.

SUMMARY

In accordance with some embodiments of the present disclosure, a systemfor electromagnetic geophysical exploration is provided. The systemincludes a first transmitter configured to transmit a first waveform ina first spectrum and a first receiver configured to sense signals in thefirst spectrum. The system also includes a second transmitter configuredto transmit a second waveform, and a second receiver configured to sensesignals in the second spectrum and minimum-coupled to the secondtransmitter. The second spectrum has a frequency higher than a frequencyof the first spectrum. The system further includes a control portionconfigured to control the first transmitter to transmit the firstwaveform in the first spectrum, and control the second transmitter totransmit the second waveform in the second spectrum after a specifiedtime delay. The control portion is also configured to receive signalssensed by the first receiver in the first spectrum, and receive signalssensed by the second receiver in the second spectrum.

In accordance with another embodiment of the present disclosure, amethod of electromagnetic geophysical exploration is provided thatincludes controlling a first transmitter to transmit a first waveform ina first spectrum. The method also includes controlling a secondtransmitter to transmit a second waveform in a second spectrum after aspecified time delay. The second spectrum has a frequency higher than afrequency of the first spectrum. The method further includes receivingsignals sensed by a first receiver in the first spectrum, and receivingsignals sensed by a second receiver in the second spectrum. The secondreceiver configured to be minimum-coupled to the second transmitter.

In accordance with another embodiment of the present disclosure, anon-transitory computer-readable medium includes instructions that, whenexecuted by a processor, cause the processor to control a firsttransmitter to transmit a first waveform in a first spectrum, andcontrol a second transmitter to transmit a second waveform in a secondspectrum after a specified time delay. The second spectrum has afrequency higher than a frequency of the first spectrum. The processoris also caused to receive signals sensed by a first receiver in thefirst spectrum, and receive signals sensed by a second receiver in thesecond spectrum. The second receiver configured to be minimum-coupled tothe second transmitter.

In accordance to yet another embodiment, there is an electromagneticgeophysical exploration system that includes a firsttransmitter-receiver pair that includes a first transmitter configuredto transmit a first waveform in a first spectrum and a first receiverconfigured to sense signals in the first spectrum; and a secondtransmitter-receiver pair that includes a second transmitter configuredto transmit a second waveform in a second spectrum, the second spectrumhaving a frequency higher than a frequency of the first spectrum, and asecond receiver configured to sense signals in the second spectrum. Thesecond receiver is configured to be minimum-coupled to the secondtransmitter, a transmitter coil of the second transmitter is disposedbetween an inner receiver coil and an outer receiver coil of the secondreceiver, and the inner receiver coil and the outer receiver coil of thesecond receiver surround the transmitter coil of the second transmitter.The control portion is configured to control the first transmitter totransmit the first waveform with a first given strength or shape in thefirst spectrum, and control the second transmitter to transmit thesecond waveform with a second given strength or shape in the secondspectrum, after a specified time delay. The electromagnetic geophysicalexploration system is configured to be airborne.

In accordance with another embodiment, there is a method ofelectromagnetic geophysical exploration that includes a step ofcontrolling with a control portion a first transmitter to transmit afirst waveform with a first given strength or shape in a first spectrum;a step of controlling with the control portion a second transmitter totransmit a second waveform with a second given strength or shape in asecond spectrum, after a specified time delay, the second spectrumhaving a frequency higher than a frequency of the first spectrum; a stepof receiving signals sensed by a first receiver in the first spectrum;and a step of receiving signals sensed by a second receiver in thesecond spectrum, the second receiver configured to be minimum-coupled tothe second transmitter. A transmitter coil of the second transmitter isdisposed between an inner receiver coil and an outer receiver coil ofthe second receiver, and the inner receiver coil and the outer receivercoil of the second receiver surround the transmitter coil of the secondtransmitter, and the first transmitter, the second transmitter, thefirst receiver and the second receiver are airborne.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and itsfeatures and advantages, reference is now made to the followingdescription, taken in conjunction with the accompanying drawings, whichmay include drawings that are not to scale and wherein like referencenumbers indicate like features, in which:

FIG. 1 illustrates a multiple bandwidth electromagnetic geophysicalexploration system with an airplane in accordance with some embodimentsof the present disclosure;

FIG. 2A illustrates a multiple bandwidth electromagnetic geophysicalexploration system with a helicopter in accordance with some embodimentsof the present disclosure, and FIG. 2B illustrates a configuration ofthe high-frequency transmitter-receiver pair;

FIGS. 3 and 4 illustrate graphs of exemplary waveforms for survey cyclesin accordance with some embodiments of the present disclosure;

FIG. 5 illustrates a flow chart of an example method for multiplebandwidth electromagnetic geophysical exploration in accordance withsome embodiments of the present disclosure; and

FIG. 6 illustrates a schematic diagram of an example system that can beused for multiple bandwidth electromagnetic geophysical exploration inaccordance with some embodiments of the present disclosure.

DETAILED DESCRIPTION

The following description of the exemplary embodiments refers to theaccompanying drawings. The same reference numbers in different drawingsidentify the same or similar elements.

The limitations of the surveying or geophysical exploration systems andmethods described above have led to the present disclosure of thesystems and methods employing multiple broadband systems (combined in away that they do not interfere) in an attempt to provide differentfrequency ranges, as well as providing some focusing of theelectromagnetic (EM) spectrum at the particular frequency ranges, withone part of the system transmitting low-frequency energy and measuringlow-frequency signals and another part of the system transmitting andmeasuring high-frequency signals. A consideration for thisdifferentiation in multiple systems is the design trade-offs describedabove: the physical geometry and electronics required to optimizemeasurements at one end of the EM spectrum (for example, low frequency)degrades performance at the other end of the spectrum (for example, highfrequency). Conceivably, low-frequency and high-frequency bandwidthsystems could be designed to be flown independently, however, thisapproach leads to errors relating to inconsistencies in position, heightand phase leading to suboptimal interpretation of the underlying geologyand additional costs. Thus, the present disclosure employs both of thesesystems in a single geophysical survey platform.

As described above, an EM survey system that uses multiple broadbandsystems to detect both low frequencies and high frequencies in the EMspectrum is expensive and can cause degraded performance at either endof the EM spectrum. Therefore, according to embodiments of the presentdisclosure, systems and methods are presented that provide multiplebandwidth EM surveying. An EM survey system of the present disclosure iscapable of measuring both the low-frequency and high-frequency portionsof the EM spectrum with improved data quality, for example, the data mayhave a higher signal to noise ratio. Measurements may be made utilizingtwo or more distinct transmitter-receiver pairs. Each of the pairs maybe optimized to a particular frequency band of interest, therebyproviding an improved data set at all frequencies.

High-frequency EM data can be used to detect and characterize subsurfacetargets with high electrical resistivity, and/or to characterizenear-surface layering—layers within, for example, approximately 165 feetor 50 meters of the earth's surface—while also discriminating thinlayers within that depth. In some embodiments, to generatehigh-frequency EM energy a signal with a fast turn-off may be used inthe transmitter. To achieve fast turn-off, the transmitter may be smallwith few turns, both of which limit dipole moment (such as, the strengthof the transmitted signal) and exploration depth. EM energy emitted by atransmitter may also be referred to as an “EM signal” or “EM wave.” Inaddition, on-time measurements during the transmitter on-time may bemade to enhance high-frequency energy. For example, when the transmitteris transmitting a waveform, a receiver may be configured to sense returnsignals approximately simultaneously.

To detect electrically conductive targets or characterize the subsurfacebeyond the near surface, a low-frequency EM excitation may be used. Insome embodiments, to generate low-frequency EM energy a transmitter maybe operated with large dipole moment. As such, to characterize thesubsurface at depths beyond the near-surface and maximize dipole moment,a transmitter with a large area (for example, physical area or numerousturns) may be used. In addition, many on-time measurements can be madeduring the ramp-up or ramp-off of the transmitter waveform to extend theoverall bandwidth of the system.

In some embodiments, the transmitter transmitting the low-frequencyenergy may be constructed as a large-diameter loop, for example on theorder of approximately 25 to 50 meters and may comprise one or moreturns of wire, for example approximately one to ten turns, or any othersuitable size or number of turns as appropriate for the implementation.The transmitter loop may be towed beneath a helicopter or other aircraftor be strung around a fixed wing aircraft or some other airbornevehicle. The helicopter, fixed-wing aircraft, or other airborne vehiclemay be manned or unmanned. In some embodiments, the low-frequencytransmitter may transmit a half-sine waveform with a low base frequency,for example, on the order of approximately three to thirty hertz (Hz)with amplitude between approximately 500 to 2,000 amperes and a useabletransmitted bandwidth of approximately three to 10,000 Hz. In someembodiments, the low-frequency receiver may have a larger coil diameterand a greater number of turns providing greater sensitivity at lowfrequencies. For example, a low-frequency coil may include approximately100 turns wound on an approximately one meter diameter former resultingin a turns-area product of approximately 79 square meters (m²) withnoise levels approaching approximately 0.01 nanovolts per square meter(nV/m²) and a receiver bandwidth of approximately one to 10,000 Hz. Thereceiver may be placed concentrically to the transmitter or placed atsome distance from the transmitter, for example, on the order ofapproximately ten to 200 meters; however, in some embodiments, thereceiver-transmitter separation may be greater than approximately 200meters. The receiver coils may be attached with some form of softsuspension system which serves to translate any high-frequency vibrationor motion into a slow, low-frequency motion below approximately five Hz,or preferably below approximately one Hz.

In some embodiments, the transmitter transmitting the high-frequencyenergy will be constructed as a smaller diameter loop on the order ofapproximately one to ten meters and may comprise only a few turns ofwire, for example, approximately one or two turns. In some embodiments,the high-frequency transmitter may transmit a square waveform ortrapezoidal waveform with a steep off-ramp such that the waveform is aclose approximation to a square waveform resulting in significant energyat high frequencies. The current amplitude of this waveform may be, forexample, on order of approximately ten to 400 amperes allowing ramptimes of approximately two to forty microseconds (μs) and a useabletransmitted bandwidth of approximately 1,000 to 100,000 Hz. There is animplicit tradeoff between minimizing ramp time and maximizing peakcurrent. The high-frequency receiver may be rigidly affixed in some wayto the high-frequency transmitter such that the electromagnetic couplingis kept constant or as close to constant as possible, and thehigh-frequency receiver may be null-coupled to the transmitter, eitherby locating the receiver at a suitable geometric location orminimum-coupled by wrapping the high-frequency receiver around thehigh-frequency transmitter wires. There are many ways to achieveminimum-coupling which may be implemented. As a rigid connection maymaintain a constant or near-constant coupling, the receiver may notemploy a suspension system.

A high-frequency coil for a high-frequency receiver may include, forexample, approximately ten turns wound on an approximately one meterdiameter former resulting in a turns area product of 7.9 m² with noiselevels approaching approximately one nV/m² and a useable bandwidth ofapproximately 1,000 to 300,000 Hz. Further, the high-frequencytransmitter may be separated from nearby air vehicles in order to reducenoise from coupling between the system and the vehicle. In someembodiments, the transmitter-receiver pair may be towed below or belowand behind a fixed-wing aircraft or balloon or other airborne vehicle,such that it is separated by some distance from the aircraft and thelow-frequency transmitter. In an embodiment with a helicopter, thehigh-frequency transmitter and receiver may be placed concentric to thelow-frequency transmitter and both may be towed below the helicopter.The low-frequency receiver may then be placed concentric to thehigh-frequency transmitter or at a location null-coupled to thelow-frequency transmitter or at some other position along the mechanismtowing the low-frequency transmitter.

Thus, the low-frequency portion of the system may comprise a large areatransmitter with multiple turns and large transmitter current tomaximize transmitter moment with low base frequency; and a half-sinewaveform measured with a large turns-area receiver with a softsuspension at some distance from the transmitter and air vehicle. Thehigh-frequency portion of the system may comprise a small areatransmitter with only one or two turns and small transmitter current tominimize current turn-off time and system transients; and employ atrapezoid or square waveform which can have a higher base frequency. Thehigh-frequency response may be measured using a small turns-areareceiver with a rigidly connected receiver (for example, one that isminimum or null-coupled). Further, there may be significant overlap ofbandwidth between the low and the high-frequency system capabilities foreffective coverage.

According to some embodiments, an EM survey system consisting of atleast two distinct transmitter and receiver sets is employed. Eachtransmitter-receiver set may be arranged to transmit and receive adifferent bandwidth of the EM spectrum. In some embodiments, thetransmitter-receiver pairs may be arranged to measure a partiallyoverlapping bandwidth of the EM spectrum. For example, onetransmitter-receiver set may be optimized to transmit and receive EMsignals of relatively high-frequency while another transmitter-receiverset may be optimized to transmit and receive relatively low-frequencysignals. The bandwidth of each transmitter-receiver pair may beoptimized such that there is some degree of frequency overlap so thatwhen combined, the dataset is continuous. The overlap between thefrequency bandwidth of each transmitter-receiver pair may aid inacquiring EM data covering the range of shallow to mid to deepsubsurface formations. Thus, at least two distinct transmitter andreceiver sets allow for optimization of each set to transmit and receivea desired frequency band of EM energy.

In some embodiments, a variety of methods may be employed to tailor thebandwidth of each transmitter-receiver set. For example, to optimizemeasurement of low-frequency information, the geometry of thetransmitter wire may be designed to consist of a large area or a largenumber of turns or permit conduction of a large amount of current.Conversely, to optimize measurement of high-frequency energy, thetransmitter wire may be designed to consist of fewer turns or lesseffective area.

The receiver measuring low-frequency information may be designed toconsist of a large effective area or large number of turns or employ amagnetic core. Conversely, the receiver measuring high-frequencyinformation may consist of smaller effective area or fewer turns. Thegeometric construction of the receiver measuring high-frequencyinformation may also be optimized for measurement of high-frequencysignals.

Thus, the low-frequency transmitter may have a predetermined geometryand a predetermined number of turns to operate in the particularlow-frequency spectrum. Similarly, the high-frequency transmitter mayhave a predetermined geometry and a predetermined number of turns tooperate in the particular high-frequency spectrum. Moreover, each of thelow-frequency receiver and the high-frequency receiver may have aparticular geometry that is optimized to make measurements for theparticular respective frequency spectrums in which each of thelow-frequency transmitter and high-frequency transmitter operate.

In addition, the receiver for the measurement of high-frequency signalsmay be disposed at an electromagnetic null location of the transmitterdesigned for high-frequency energy excitation. An electromagnetic nulllocation may be a location at which the response of the receiver isnulled to the primary EM field of the transmitter. An electromagneticnull may be accomplished, for example, by minimizing coupling betweenthe transmitter and receiver. For example, minimizing coupling betweenthe transmitter and receiver may be achieved by disposing the receiverhalf-inside and half-outside the transmitter loop or by disposing thereceiver at the appropriate angle away from the transmitter loop.

FIG. 1 illustrates a multiple bandwidth electromagnetic geophysicalexploration system 150 with airplane 100 in accordance with someembodiments of the present disclosure. FIG. 1 depicts an exemplaryphysical embodiment of a multiple bandwidth EM geophysical explorationsystem 150, also referred to as “system” 150. In some embodiments,system 150 may include airplane 100 that tows two or more transmittersand receivers. Although illustrated with one airplane 100, multipleairplanes 100 may be utilized in system 150 or a multitude of receivers112 with different bandwidths and/or effective areas may be utilized.

In some embodiments, one or multiple transmitter-receiver pairs may bedesigned to measure low-frequency signals. For example, low-frequencytransmitter 110 is positioned around airplane 100 and consists of alarge area. Low-frequency transmitter 110 may include a coil surroundingairplane 100 and secured to a plurality of different points on the bodyof airplane 100.

Low-frequency receiver 112 may be optimized for low-frequency energy.Low-frequency receiver 112 may be towed behind or below low-frequencytransmitter 110. Low-frequency receiver 112 may be towed by the sameairplane 100 that tows low-frequency transmitter 110. Alternatively,low-frequency receiver 112 may be towed by a different airplane.Low-frequency receiver 112 may be designed to measure low-frequencysignals by increasing turns-area (for example, a larger area for turnsof wire on the coil) and increasing electronic gain.

In some embodiments, a transmitter-receiver pair may be designed tomeasure high-frequency signals. For example, high-frequency transmitter120 may be small in area, allowing a fast transmitter current turn-off.High-frequency receiver 122 may be null-coupled to high-frequencytransmitter 120, which reduces transmitter transients and allowsmeasurement at very early off-times. Similarly, high-frequency receiver122 may be optimized to measure high-frequency signals.

As depicted in FIG. 1, high-frequency transmitter 120 may be smallerthan low-frequency transmitter 110 and may be disposed below and aspecified distance away from low-frequency transmitter 110.High-frequency receiver 122 may be arranged in a manner, as describedabove, to be null-coupled to high-frequency transmitter 120.

To increase the amount information acquired in the survey, a pluralityof low-frequency receivers 112 may be utilized. For example, FIG. 1depicts a second low-frequency receiver 112 towed from airplane 100.Further, although an embodiment is depicted and described with airplane100, the systems and methods of the present disclosure could also beapplied to land-based or marine-based systems.

The low-frequency transmitter-receiver pair, low-frequency transmitter110 and low-frequency receiver(s) 112, may be designed for high-moment,low base frequency operation. The low-frequency transmitter-receiverpair may be configured to optimize performance at low frequencies by,for example, increasing coil gain electronically or by turns-area or bya magnetic core, and may include more suspension and a non-rigidconnection to reduce resonant frequency of system motion.

In contrast, the high-frequency transmitter-receiver pair may have arigid connection with no suspension system. In the high-frequencytransmitter-receiver pair, measurement at early delay times (forexample, times shortly after turn-off) is important for obtainingnear-surface information and thus, the self-response time of thehigh-frequency receiver and the time length of the signal turn-off needto be short. The design and configuration of the high-frequency receivermay minimize its self-response time, for example, by having a smallturns-area. Further, by having fewer turns the transmitter current maybe turned off quickly, such that higher frequency energy is excited intothe ground. Moreover, by null-coupling the high-frequencytransmitter-receiver pair (for example, disposing the receiver in a“null” position with respect to the transmitter), the transmittertransients (for example, transient energy) may be minimized to reducetheir effect on the measured data.

The signals sensed by low-frequency receivers 112 and the signals sensedby the high-frequency receiver 122 may be received, for example, at aprocessing or control portion 160, as depicted in FIG. 1. Such aprocessing or control portion 160 may be disposed onboard an aircraft(for example, airplane 100 or helicopter 200, discussed with referenceto FIG. 2A) or other vehicle or machine used for surveying or towed bythe aircraft. Alternatively, such a processing or control portion 160may be disposed remotely.

FIG. 2A illustrates a multiple bandwidth electromagnetic geophysicalexploration system 250 with a helicopter 200 in accordance with someembodiments of the present disclosure. As depicted in FIG. 2A, alow-frequency transmitter loop 210 may be disposed around atransmitter-receiver unit that may be towed by helicopter 200.Low-frequency transmitter 210 may be a substantially circular coil,forming a loop around the transmitter-receiver unit that may besuspended from helicopter 200. Alternatively, low-frequency transmitter210 may be a square, diamond, rectangle, or other polygonal shape. Alow-frequency receiver 212 is disposed substantially at the center ofthe loop formed by low-frequency transmitter 210. Another embodiment maybe to place the low-frequency receiver at the apex of the set of cablessupporting the low-frequency transmitter loop 210 or set of cablessupporting high-frequency loops. Similar to the low-frequencytransmitter-receiver pair described above for airplane 100 depicted inFIG. 1, low-frequency transmitter 210 and low-frequency receiver 212 areconfigured to be optimized to operate in a low-frequency bandwidth.

A high-frequency transmitter 220 and a high-frequency receiver 222 maybe minimum-coupled. For example, as described above, high-frequencyreceiver may be arranged with respect to high-frequency transmitter 220to be proximate to a “null” location. Further, high-frequencytransmitter 220 and high-frequency receiver 222 may form a substantiallycircular coil and may be disposed to form a loop surroundinglow-frequency receiver 212, but with a smaller diameter thanlow-frequency transmitter 210. For example, the null-coupledhigh-frequency transmitter-receiver pair 220/222 may be disposed suchthat a transmitter coil 220A for high-frequency transmitter 220 isdisposed such that an inner receiver coil 222 i and an outer receivercoil 222 o for high-frequency receiver 222 surround the transmitter coil220A for high-frequency transmitter 220, as shown in FIG. 2B. In otherwords, the transmitter coil 220A for high-frequency transmitter 220 maybe disposed between an inner receiver coil 222 i and an outer receivercoil 222 o for high-frequency receiver 222, with each of the coils beingsubstantially circular, as illustrated in FIG. 2B. In some embodiments,any of low-frequency transmitter 210, low-frequency receiver 212,high-frequency transmitter 220, and high-frequency receiver 222 may bepolygonal, or any other suitable shape based on implementation.

In operation, each transmitter may generate an electromagneticexcitation. The excitation of each transmitter loop may occursimultaneously or allow for some specified time delay betweenexcitations. Excitation of each transmitter may be performed at nearbylocations such that high-frequency and low-frequency information isobtained for the same area. The bandwidth or frequency spectrum for thehigh-frequency transmitter-receiver pair includes frequencies greaterthan the low-frequency transmitter-receiver pair, but the bandwidths mayoverlap each other to allow for a continuous bandwidth fromlow-frequency to high-frequency to be implemented. Thus, for example,the high-frequency transmitter-receiver pair may be configured to senseearth properties from surface to approximately 100-200 meters depth,while the low-frequency transmitter-receiver pair may be configured tosense earth properties from approximately 50 meters beneath the surfaceto approximately 600-800 meters or greater beneath the surface.

FIGS. 3 and 4 illustrate graphs 300 and 400 of exemplary currentwaveforms 310 and 410 for a transmitted cycle in accordance with someembodiments of the present disclosure. The excitation waveform for thetwo different transmitters 110 and 120 may be of different shape,amplitude or repetition frequency, or a combination thereof. Forexample, low-frequency transmitters 110 and 210, discussed withreference to FIGS. 1 and 2, respectively, may emit a waveform with halfsine pulses. As another example, high-frequency transmitters 120 and 220may emit a waveform with substantially square pulses or trapezoidalpulses. As depicted in FIG. 3, a single cycle may include, for example,positive half sine pulses and positive square pulses, separated by anoff time and negative half sine pulses and negative square pulses.Equally valid would be a series of positive and negative half sinepulses followed by a series of positive and negative square ortrapezoidal pulses. As depicted in FIG. 4, a single cycle may include,for example, positive half sine pulses and positive trapezoidal pulses,separated by an off time and negative half sine pulses and negativetrapezoidal pulses.

Data from each survey may be used independently for processing,interpretation, modelling, or transforming from data to electricalproperties or to geology. By using at least two transmitter-receiverpairs, a system with two separate, overlapping bandwidths may bettermeasure a wider range of frequencies. In some embodiments, the twotransmitter-receiver pairs may be based on a single transmitter enabledto emit both low-frequency and high-frequency signals that may be pairedseparately with two different receivers. Accordingly, more accurate androbust information regarding earth properties may be obtained bycombining the data from the two transmitter-receiver pairs andperforming simultaneous processing, modelling, or transforming the datato electrical properties or geology. Further, modelling or interpretingdata from one of the transmitter-receiver pairs and subsequent modellingor interpretation of data from the other transmitter-receiver set basedon the results from the first set may also improve the interpretation,model, or product. For example, the data from each of the low-frequencyand high-frequency transmitter-receiver pairs may be cross-correlatedfor improved signal noise reduction. Additionally the overlappingbandwidths could be used to calibrate the overall system response.

FIG. 5 illustrates a flow chart of an example method 500 for multiplebandwidth electromagnetic geophysical exploration in accordance withsome embodiments of the present disclosure. The steps of method 500 areperformed by a user, various computer programs, models configured toprocess or analyze geophysical data, and combinations thereof. Theprograms and models include instructions stored on a computer readablemedium and operable to perform, when executed, one or more of the stepsdescribed below. The computer readable media includes any system,apparatus or device configured to store and retrieve programs orinstructions such as a hard disk drive, a compact disc, flash memory, orany other suitable device. The programs and models are configured todirect a processor or other suitable unit to retrieve and execute theinstructions from the computer readable media. Collectively, the user orcomputer programs and models used to process and analyze geophysicaldata may be referred to as a “computing system.” For illustrativepurposes, method 500 is described with respect to data based on controlportion 160 of FIGS. 1 and 2. A particular survey may include manycycles of transmitting and receiving electromagnetic signals and is notlimited to any order of processes described herein.

At step 510, a control portion may control a low-frequency transmitterto transmit a low-frequency waveform in a lower frequency spectrum. Atstep 520, the control portion may control an high-frequency transmitterto transmit a waveform in a higher frequency spectrum after a specifiedtime. The frequency spectrum emitted by the high-frequency transmittermay be higher than the frequency spectrum emitted by the low-frequencytransmitter, or may overlap the frequency spectrum emitted by thelow-frequency transmitter. In other words, at least a portion of thehigher frequency spectrum is greater than the lower frequency spectrum.Further, in some embodiments, one transmitter may be utilized totransmit both the high-frequency and the low-frequency signals. At step530, the control portion receives signals that are sensed by alow-frequency receiver and an high-frequency receiver. In someembodiments, the receivers may receive signals during the time that thetransmitter is transmitting a waveform, for example, during the on-timeof the transmitters. At step 540, the control portion may store and/orprocess the received signals. Alternatively, the raw data from thereceived signals may be stored for processing at a later time. At step550, the control portion may analyze the received data and detect earthproperties from surface to varying depths beneath the surface. In method500, the steps may be performed in any appropriate order. For example,step 530 may be performed concurrently with steps 510 and 520. Asanother example, step 520 may occur before or concurrently with step510.

As described above, the design requirements for efficient transmittingand measuring of low and high-frequency energy and signals requirestradeoffs and the present disclosure represents an advantageous methodto perform both simultaneously. Embodiments of the present disclosureenable two independent time domain systems with different bandwidths,such that multiple transmitters and multiple receivers may beimplemented in a single system to achieve optimized multiple bandwidthelectromagnetic surveying in the single system. The waveform transmittedby the low-frequency transmitter and the waveform transmitted by thehigh-frequency transmitter may be synchronized in time to accuratelymeasure in both the low-frequency spectrum and the high-frequencyspectrum.

An advantage of the present disclosure is overcoming certainshortcomings of geophysical systems that are either broadband oroptimized to one of deep or shallow exploration. The present disclosureeffectively achieves two optimized systems; one for near-surfaceresolution and the other for deep exploration; in a single system. Thepresent disclosure achieves a commercial advantage by capturing allrelevant information about earth properties of prospective land withoutthe need to conduct multiple surveys or the limitations of differentsystems optimized for different depths and targets. Another advantage ofthe present disclosure is that it provides increased sensitivity withrespect to vertical resolution or spatial resolution and depth ofexploration. An advantage of the distinct transmitter-receiver sets ofthe present disclosure may be realized because the electrical componentsfor measurement of high and low-frequency energy may be different. Afurther advantage of the distinct transmitter-receiver sets of thepresent disclosure may be realized by the physical separation in spaceof the two transmitter-receiver pairs and by a separation in time of theexcitation from each of the transmitters.

FIG. 6 illustrates a schematic diagram of an example system 600 that canbe used for multiple bandwidth electromagnetic geophysical explorationin accordance with some embodiments of the present disclosure. System600 includes one or more transmitters 602, one or more receivers 604,and computing system 606, which are communicatively coupled via network608. Computing system 606 may include some or all components of controlportion 160 discussed with reference to FIGS. 1 and 2.

Computing system 606 can operate in conjunction with transmitters 602and receivers 604 having any structure, configuration, or function.Transmitters 602 may include low-frequency transmitters andhigh-frequency transmitters, and receivers 604 may include low-frequencyreceivers and high-frequency receivers. Further, a positioning system,such as a global positioning system (GPS, GLONASS, etc.), may beutilized to locate or time-correlate transmitters 602 and receivers 604.

Computing system 606 may include any instrumentality or aggregation ofinstrumentalities operable to compute, classify, process, transmit,receive, store, display, record, or utilize any form of information,intelligence, or data. For example, computing system 606 may be one ormore mainframe servers, desktop computers, laptops, cloud computingsystems, storage devices, or any other suitable devices and may vary insize, shape, performance, functionality, and price. Computing system 606may include random access memory (RAM), one or more processing resourcessuch as a central processing unit (CPU) or hardware or software controllogic, or other types of volatile or non-volatile memory. Additionalcomponents of computing system 606 may include one or more disk drives,one or more network ports for communicating with external devices,various input and output (I/O) devices, such as a keyboard, a mouse, anda video display. Computing system 606 may be configured to permitcommunication over any type of network 608. Network 608 can be awireless network, a local area network (LAN), a wide area network (WAN)such as the Internet, or any other suitable type of network.

Network interface 610 represents any suitable device operable to receiveinformation from network 608, transmit information through network 608,perform suitable processing of information, communicate with otherdevices, or any combination thereof. Network interface 610 may be anyport or connection, real or virtual, including any suitable hardwareand/or software (including protocol conversion and data processingcapabilities) that communicates through a LAN, WAN, or othercommunication system. This communication allows computing system 606 toexchange information with network 608, other computing systems 606,transmitters 602, receivers 604, or other components of system 600.Computing system 606 may have any suitable number, type, and/orconfiguration of network interface 610.

Processor 612 communicatively couples to network interface 610 andmemory 614 and controls the operation and administration of computingsystem 606 by processing information received from network interface 610and memory 614. Processor 612 includes any hardware and/or software thatoperates to control and process information. In some embodiments,processor 612 may be a programmable logic device, a microcontroller, amicroprocessor, any suitable processing device, or any suitablecombination of the preceding. Computing system 606 may have any suitablenumber, type, and/or configuration of processor 612. Processor 612 mayexecute one or more sets of instructions to implement multiple bandwidthelectromagnetic surveying, including the steps described above withrespect to FIG. 5. Processor 612 may also execute any other suitableprograms to facilitate the generation of broadband composite images suchas, for example, user interface software to present one or more GUIs toa user.

Memory 614 stores, either permanently or temporarily, data, operationalsoftware, or other information for processor 612, other components ofcomputing system 606, or other components of system 600. Memory 614includes any one or a combination of volatile or nonvolatile local orremote devices suitable for storing information. For example, memory 614may include random access memory (RAM), read only memory (ROM), flashmemory, magnetic storage devices, optical storage devices, networkstorage devices, cloud storage devices, solid-state devices, externalstorage devices, any other suitable information storage device, or acombination of these devices. Memory 614 may store information in one ormore databases, file systems, tree structures, any other suitablestorage system, or any combination thereof. Furthermore, different typesof information stored in memory 614 may use any of these storagesystems. Moreover, any information stored in memory may be encrypted orunencrypted, compressed or uncompressed, and static or editable.Computing system 606 may have any suitable number, type, and/orconfiguration of memory 614. Memory 614 may include any suitableinformation for use in the operation of computing system 606. Forexample, memory 614 may store computer-executable instructions operableto perform the steps discussed above with respect to FIG. 5 whenexecuted by processor 612. Memory 614 may also store any seismic data orrelated data such as, for example, raw seismic data, reconstructedsignals, velocity models, seismic images, or any other suitableinformation.

The foregoing detailed description does not limit the disclosure.Instead, the scope of the disclosure is defined by the appended claims.The described embodiments are not limited to the disclosedconfigurations, and may be extended to other arrangements.

Reference throughout the specification to “one embodiment” or “anembodiment” means that a particular feature, structure or characteristicdescribed in connection with an embodiment is included in at least oneembodiment of the subject matter disclosed. Thus, the appearance of thephrases “in one embodiment” or “in an embodiment” in various placesthroughout the specification is not necessarily referring to the sameembodiment. Further, the particular features, structures orcharacteristics may be combined in any suitable manner in one or moreembodiments.

Herein, “or” is inclusive and not exclusive, unless expressly indicatedotherwise or indicated otherwise by context. Therefore, herein, “A or B”means “A, B, or both,” unless expressly indicated otherwise or indicatedotherwise by context. Moreover, “and” is both joint and several, unlessexpressly indicated otherwise or indicated otherwise by context.Therefore, herein, “A and B” means “A and B, jointly or severally,”unless expressly indicated otherwise or indicated otherwise by context.

This disclosure encompasses all changes, substitutions, variations,alterations, and modifications to the example embodiments herein that aperson having ordinary skill in the art would comprehend. Similarly,where appropriate, the appended claims encompass all changes,substitutions, variations, alterations, and modifications to the exampleembodiments herein that a person having ordinary skill in the art wouldcomprehend. Moreover, reference in the appended claims to an apparatusor system or a component of an apparatus or system being adapted to,arranged to, capable of, configured to, enabled to, operable to, oroperative to perform a particular function encompasses that apparatus,system, component, whether or not it or that particular function isactivated, turned on, or unlocked, as long as that apparatus, system, orcomponent is so adapted, arranged, capable, configured, enabled,operable, or operative. For example, a receiver does not have to beturned on but must be configured to receive reflected energy.

Any of the steps, operations, or processes described herein may beperformed or implemented with one or more hardware or software modules,alone or in combination with other devices. In one embodiment, asoftware module is implemented with a computer program productcomprising a computer-readable medium containing computer program code,which can be executed by a computer processor for performing any or allof the steps, operations, or processes described. For example, thetransmitting waveform, receiving sensed signals, and processing ofreceived signals processes may be performed through execution ofcomputer program code in a computer-readable medium.

Embodiments of the present disclosure may also relate to an apparatusfor performing the operations herein. This apparatus may be speciallyconstructed for the required purposes, and/or it may comprise ageneral-purpose computing device selectively activated or reconfiguredby a computer program stored in the computer. Such a computer programmay be stored in a tangible computer-readable storage medium or any typeof media suitable for storing electronic instructions, and coupled to acomputer system bus. Furthermore, any computing systems referred to inthe specification may include a single processor or may be architecturesemploying multiple processor designs for increased computing capability.

Although the present disclosure has been described with severalembodiments, a myriad of changes, variations, alterations,transformations, and modifications may be suggested to one skilled inthe art, and it is intended that the present disclosure encompass suchchanges, variations, alterations, transformations, and modifications asfall within the scope of the appended claims. Moreover, while thepresent disclosure has been described with respect to variousembodiments, it is fully expected that the teachings of the presentdisclosure may be combined in a single embodiment as appropriate.Instead, the scope of the present disclosure is defined by the appendedclaims.

What is claimed is:
 1. An electromagnetic geophysical exploration systemcomprising: a first transmitter-receiver pair that includes a firsttransmitter configured to transmit a first waveform in a first spectrumand a first receiver configured to sense signals in the first spectrum;a second transmitter-receiver pair that includes a second transmitterconfigured to transmit a second waveform in a second spectrum, thesecond spectrum having a frequency higher than a frequency of the firstspectrum, and a second receiver configured to sense signals in thesecond spectrum, wherein the second receiver is configured to beminimum-coupled to the second transmitter, a transmitter coil of thesecond transmitter is disposed between an inner receiver coil and anouter receiver coil of the second receiver, and the inner receiver coiland the outer receiver coil of the second receiver surround thetransmitter coil of the second transmitter; and a control portionconfigured to: control the first transmitter to transmit the firstwaveform with a first given strength or shape in the first spectrum, andcontrol the second transmitter to transmit the second waveform with asecond given strength or shape in the second spectrum, after a specifiedtime delay, wherein the electromagnetic geophysical exploration systemis configured to be airborne.
 2. The system of claim 1, wherein thecontrol portion is further configured to store the received signals. 3.The system of claim 2, wherein the control portion is further configuredto process the stored signals to analyze a subsurface formation.
 4. Thesystem of claim 1, wherein the first waveform includes a differentwaveform shape from the second waveform.
 5. The system of claim 1,wherein the first spectrum and the second spectrum include anoverlapping bandwidth portion.
 6. The system of claim 1, wherein thesecond transmitter is configured to minimize a turn-off time for thesecond waveform.
 7. The system of claim 1, wherein the first transmitteris a low-frequency transmitter and the first receiver is a low-frequencyreceiver.
 8. The system of claim 1, wherein the second transmitter is ahigh-frequency transmitter and the second receiver is a high-frequencyreceiver.
 9. The system of claim 1, wherein the signals sensed by thefirst receiver in the first spectrum are sensed during transmission ofthe first waveform by the first transmitter.
 10. A method ofelectromagnetic geophysical exploration, comprising: controlling with acontrol portion a first transmitter to transmit a first waveform with afirst given strength or shape in a first spectrum; controlling with thecontrol portion a second transmitter to transmit a second waveform witha second given strength or shape in a second spectrum, after a specifiedtime delay, the second spectrum having a frequency higher than afrequency of the first spectrum; receiving signals sensed by a firstreceiver in the first spectrum; and receiving signals sensed by a secondreceiver in the second spectrum, the second receiver configured to beminimum-coupled to the second transmitter, wherein a transmitter coil ofthe second transmitter is disposed between an inner receiver coil and anouter receiver coil of the second receiver, and the inner receiver coiland the outer receiver coil of the second receiver surround thetransmitter coil of the second transmitter, and wherein the firsttransmitter, the second transmitter, the first receiver and the secondreceiver are airborne.
 11. The method of claim 10, further comprisingstoring the received signals.
 12. The method of claim 11, furthercomprising processing the stored signals to analyze a subsurfaceformation.
 13. The method of claim 10, wherein the first waveformincludes a different waveform shape from the second waveform.
 14. Themethod of claim 10, wherein the first spectrum and the second spectruminclude an overlapping bandwidth portion.
 15. The method of claim 10,wherein the second transmitter is configured to minimize a turn-off timefor the second waveform.